### Synopsis

In the previous acoustics tutorial, you were introduced to the Statistical Reverberation Time (RT) graph in Ecotect. This tutorial explores the different type of algorithms Ecotect uses to generate this graph, and when it might be appropriate to use each one.

##### Duration

You will need about 20 minutes to complete this tutorial.

##### Training and Accreditation

Successfully completing this tutorial fulfills **one** of the practical skills required for completing **Level 1** of the **Acoustic Design** training module.

##### Resources Required

To complete this tutorial, you will need the *AcousticRays.eco* example file. This can be found in the 'Examples' folder located in the folder where Ecotect is installed on your computer.

### Tutorial

- Open the
*AcousticRays.eco*file and generate a statistical RT graph as outlined in the previous tutorial. - Ecotect uses three types of algorithms for RT analysis - Sabine, Norris-Eyring and Millington-Sette. You can use the
*Calculation*list boxes to select which one to display or highlight. - The Sabine algorithm for calculating RT assumes that sound decays continuously and smoothly, as would be found in a room where there is little variation in the absorption values of the room's surfaces. While the Sabine algorithm can be used to quickly calculate a room's RT response, it has been shown that the Sabine algorithm becomes less accurate as the absorption values of a room's surfaces increase.
- The Norris-Eyring formula attempts to improve the accuracy of the Sabine algorithm by assuming sound decays intermittently, rather than continuously. Thus, the Norris-Eyring algorithm is more accurate when used to calculate RT in rooms where the surfaces have high absorption coefficients. However, it still assumes that all of the room's surfaces have the same absorption values, which rarely happens in most built spaces.
- When the materials of a room have a wide variety of absorption coefficients, the best predictions are obtained by using the Millington-Sette equation. However, this formula indicates that highly absorbing materials are far more effective than would actually be anticipated in influencing the reverberation time, and so may not necessarily provide a more realistic prediction of the acoustic response of a space.
- When using statistical formula to predict the RT of a space, it is important to note that all of the equations described are purely statistical in nature and, as such, neglect all of the geometric information about the room (its shape, the position of absorbing materials, the use of reflectors, etc). Thus, whilst they can closely indicate the reverberation time, they cannot be used to predict any acoustic anomalies within a room, such as discernible echoes, acoustic shadows, etc. For that, you need a detailed geometric analysis, which is covered in later tutorials.